This application claims the benefit of Korean Patent Application No. 1999 -63250, filed on Dec. 28, 1999, under 35 U.S.C. xc2xa7119, the entirety of which is hereby incorporated by reference.
1. Field of the invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a transflective LCD device and a method of manufacturing the same.
2. Description of Related Art
Until now, the cathode-ray tube (CRT) has been developed for and is mainly used for the display systems. However, the flat panel display is beginning to make its appearance due to the requirement of the small depth dimensions and the desirability of low weight and low voltage power supply. At this point, the thin film transistor-liquid crystal display (TFT-LCD) having a high resolution and small depth dimension has been developed.
In the operating principles of the TFT-LCD, when the pixel is turned ON by the switching elements, the pixel transmits the light generated from the backlight device. The switching elements are generally an amorphous silicon thin film transistor (a-Si:H TFT) which has the semiconductor layer because the amorphous silicon TFT can be formed on a low cost glass substrate at low temperature.
In general, the TFT-LCD produces the image using the light from the back light device that is positioned under the TFT-LCD panel. However, the TFT-LCD only employs 3xcx9c8% of the incident light generated from the backlight device, i.e., inefficient optical modulation.
Referring to the attached drawings, an array substrate of an LCD device that is manufactured by a conventional method will now be explained in some detail.
FIG. 1 is a graph illustrating a transmittance respectively measured after light passes through each layer of a conventional liquid crystal display device.
The two polarizers have a transmittance of 45% and, the two substrates have a transmittance of 94%. The TFT array and the pixel electrode have a transmittance of 65%, and the color filter has a transmittance of 27%. Therefore, the typical transmissive LCD device has a transmittance of about 7.4% as seen in FIG. 1, which shows a transmittance (in brightness %) after light passes through each layer of the device. For this reason, the transmissive LCD device requires a high initial brightness, and thus electric power consumption by the backlight device increases. A relatively heavy battery is needed to supply sufficient power to the backlight of such a device. Moreover, there still exists a problem that the battery can not be used for a long time.
In order to overcome the problem described above, the reflective LCD has been developed. Since the reflective LCD device uses ambient light, it is light and easy to carry. Also, the reflective LCD device is superior in aperture ratio compared to the transmissive LCD device.
FIG. 2 is a plan view illustrating a typical reflective LCD device. As shown in FIG. 2, the reflective LCD device 100 includes gate lines 6 and 8 arranged in a transverse direction, data lines 2 and 4 arranged in a longitudinal direction perpendicular to the gate lines 6 and 8, and thin film transistors (TFTs), for example, the thin film transistor xe2x80x9cSxe2x80x9d near a cross point of the gate line 8 and the data line 2. Each of the TFTs xe2x80x9cSxe2x80x9d has a gate electrode 18, a source electrode 12 and a drain electrode 14. The source electrode 12 extends from the data line 2, and the gate electrode 18 extends from the gate line 8. The reflective LCD device 100 further includes reflective electrodes 10. The reflective electrode 10 is electrically connected with the drain electrode 14 through a contact hole 16 and is made of a metal having a good reflectance.
FIG. 3 is a cross sectional view taken along the line IIIxe2x80x94III of FIG. 2. As shown in FIG. 3, the gate electrode 18 is formed on the substrate 1, and a gate insulating layer 20 is formed on the exposed surface of the substrate 1 while covering the gate electrode 18. A semiconductor layer 22 as an active area of the TFT xe2x80x9cSxe2x80x9d (see FIG. 2) is formed over the gate electrode 18. The source and drain electrodes 12 and 14 are spaced apart from each other. The source electrode 12 overlaps one end portion of the semiconductor layer 22, and the drain electrode 14 overlaps the other end portion of the semiconductor layer 22. A passivation film 24 is formed over the whole surface of the substrate 1 while covering the TFT xe2x80x9cSxe2x80x9d. The passivation film 24 has the contact hole 16 on the predetermined portion of the drain electrode 14. The reflective electrode 10 is formed on the passivation film 24 and is electrically connected with the drain electrode 14 through the contact hole 16.
As mentioned above, since the reflective LCD device uses ambient light, a battery is not necessary. By the way, the reflective LCD device has a problem in that it is affected by its surroundings. For example, the brightness of indoors-ambient light differs largely from that of outdoors. Also, even in the same location, the brightness of ambient light depends on the time of day (e.g., noon or dusk). Therefore, the reflective LCD device cannot be used at night without ambient light.
For the foregoing reasons, there is a need for a transflective LCD device that can be used during the day as well as at night.
FIG. 4 is a plan view illustrating an array substrate of a transflective liquid crystal display (LCD) device according to a conventional art. As shown in FIG. 4, the array substrate includes a gate line 50 arranged in a transverse direction, data line 60 arranged in a longitudinal direction perpendicular to the gate line 50, and a thin film transistor (TFT) arranged near the cross portion of the gate and data lines 50 and 60. The TFT has a gate electrode 52, a source electrode 62 and a drain electrode 64. The gate electrode 52 extends from the gate line 50, and the source electrode 62 extends from the data line 60. The drain electrode 64 is spaced apart from the source electrode 62. And the source electrode 62 overlaps one end portion of the gate electrode 52, and the drain electrode 64 overlaps the other end portion of the gate electrode 52. The array substrate further includes a reflective electrode 68 and a pixel electrode 70, which are formed on a region defined by the gate and data lines 50 and 60. The reflective electrode 68 and the pixel electrode 70 are electrically connected with the drain electrode 64 through contact hole 69 and 66 (see FIG. 5C). The reflective electrode 68 is made of an opaque conductive metal, and the pixel electrode 70 is made of a transparent conductive material. The reflective electrode 68 has a light transmitting hole 72 formed on a central portion thereof. The light transmitting hole 72 serves to transmit light and has a substantially rectangular shape. The pixel electrode 70 has a sufficient size to cover the light transmitting hole 72. In other words, the pixel electrode 70 covers the light transmitting hole 72.
FIGS. 5A to 5D are cross sectional views taken along the line Vxe2x80x94V of FIG. 4, illustrating a process of manufacturing the array substrate of the transflective LCD device according to the conventional art.
First, as shown in FIG. 5A, a first metal layer is deposited on a substrate 1 and patterned into the gate electrode 52. The first metal layer is made of a metal having a high corrosion resistance such as Chrome or Tungsten or having a low resistance such as Aluminum alloy.
Then, as shown in FIG. 5B, a gate insulating layer 80, a semiconductor layer 82 and the source and drain electrodes 62 and 64 are sequentially formed. The gate insulating layer 80 is formed on the exposed surface of the substrate 1 while covering the gate electrode 52. The semiconductor layer 82 is formed on the gate insulating layer 80 and over the gate electrode 52. The source electrode 62 overlaps one end portion of the semiconductor layer 82, and the drain electrode 64 overlaps the other end portion of the semiconductor layer 82. The source and drain electrodes 62 and 64 are spaced apart from each other.
Sequentially, as shown in FIG. 5C, a passivation film 84 is formed on the exposed surface of the gate insulating layer 80 while covering the source and drain electrodes 62 and 64. A portion of the passivation film 84 on the drain electrode 54 is etched to form a first contact hole 66. The passivation film 84 is made of an insulating material having a good moisture resistance and a good transmittance and preferably Silicon Nitride (SiNx) or Silicon Oxide (SiOx). Next, the pixel electrode 70 is formed on the passivation film 84 and is electrically connected with the drain electrode 64 through the first contact hole 66. The pixel electrode 70 is made of a transparent conductive metal having a good transmittance and preferably one of Indium Tin Oxide (ITO) and Indium Zinc Oxide (IZO).
After that, as shown in FIG. 5D, an inter-layer insulating film 86 is formed over the entire surface of the substrate 1 while covering the pixel electrode 70. The interlayer insulating film 86 is made of made of one of Benzocyclobutene (BCB) that has a good transmittance. A portion of the inter-layer insulating film 86 over the first contact hole 66 is etched to form a second contact hole 69. Then, the reflective electrode 68 is formed on the inter-layer insulating film 86 and is electrically connected with the pixel electrode 70. A portion of the reflective electrode 68 is etched to form the light transmitting hole 72.
FIG. 6 is a schematic cross-sectional view illustrating the operating principle of the transflective LCD device according to the conventional art. As shown in FIG. 6, the transflective LCD device includes a liquid crystal panel and a backlight device 102. The liquid crystal display panel includes lower and upper substrates 108 and 106 with an interposed liquid crystal layer 100. The upper substrate 106 has a color filter 104, and the lower substrate 108 as the array substrate has the TFT, the pixel electrode 70 and the reflective electrode 68. The reflective electrode 68 includes the light transmitting hole 72 formed therein. The inter-layer insulating film 86 is interposed between the reflective electrode 68 and the pixel electrode 70. The pixel electrode 70 covers a region corresponding to the light transmitting hole 72. The transflective LCD device further includes an upper polarizer (not shown) on the upper substrate 106 and a lower polarizer (not shown) located between the lower substrate 108 and the backlight device 102.
The transflective LCD device according to the conventional art is operated as follows.
First, in the reflective mode, the incident light 110 from the outside is reflected on the reflective electrode 68 and directs toward the upper substrate 106 again. At this time, when the electrical signals are applied to the reflective electrode 68 by the switching element (not shown), phase of the liquid crystal layer 100 varies and thus the reflected light of the incident light 110 is colored by the color filter 104 and displayed in the form of colored light. In the transmissive mode, light 112 emitted from the backlight device 102 passes through the transmitting holes 72. At this time, when the electrical signals are applied to the pixel electrode 70 by the switching element (not shown), phase of the liquid crystal layer 100 varies. Thus, the light 112 passing through the liquid crystal layer 100 is colored by the color filter 104 and displayed in the form of images with other colored lights.
FIG. 7 is an enlarged view illustrating the portion xe2x80x9cAxe2x80x9d of FIG. 5D, focused on the first and second contact holes. In the conventional art, as shown in FIG. 7, the passivation layer 84 is etched by using a photolithography process to form the first contact hole 66 through which the pixel electrode 70 contacts the drain electrode 64. And then the inter-layer insulating layer 86 is also etched to form the second contact hole 69 through which the reflective electrode 68 contacts the pixel electrode 70. Thus, the reflective electrode 68 is electrically connected to the drain electrode 64.
However, the above-mentioned process has some problems in that the photolithography process is performed twice to form the first and second contact holes and to electrically connect the reflective electrode to the drain electrode. Also, the photolithography process includes a lot of processes such as a cleaning process, an exposure process, a baking process, a developing process, etc.
Therefore, if one photolithography process is omitted, the manufacturing yields will increase and the defects caused by misalignment will decrease.
Meanwhile, due to the fact that the reflective electrode made of opaque metal is formed in the latest process step and that the reflective electrode reflects an alignment signal very well, the align key is not easily recognized during the photolithography process, i.e., misalignment occurs.
Accordingly, the present invention is directed to an array substrate of an LCD device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
To overcome the problems described above, a preferred embodiment of the present invention provides a transflective LCD device manufactured by a simplified process.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from that description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
In order to achieve the above object, the preferred embodiment of the present invention provides an array substrate of transflective liquid crystal display (LCD) device, including: a substrate having switching elements and a pixel region; a reflective plate formed on the substrate and having a light transmitting hole; a first insulating layer formed on the reflective plate while covering the light transmitting hole; a gate electrode formed on the first insulating layer over the reflective plate; a gate insulating layer formed on the first insulating layer while covering the gate electrode; an active layer formed on the gate insulating layer over the gate electrode and having a channel region; an ohmic contact layer formed on the active layer; source and drain electrodes formed on the ohmic contact layer and spaced apart from each other; a second insulating layer formed on the gate insulating layer while covering the source and drain electrode, the second insulating layer having a drain contact hole which exposes the predetermined portion of the drain electrode; and a pixel electrode formed on the second insulating layer and contacting the drain electrode through the drain contact hole.
The reflective electrode is beneficially made of a opaque conductive metal and the pixel electrode is beneficially made of the material selected from a group of consisting of Indium-Tin-Oxide (ITO) and Indium-Zinc-Oxide (IZO).
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.